Low Pt loading for high-performance fuel cell electrodes enabled by hydrogen-bonding microporous polymer binders

A key challenge for fuel cells based on phosphoric acid doped polybenzimidazole membranes is the high Pt loading, which is required due to the low electrode performance owing to the poor mass transport and severe Pt poisoning via acid absorption on the Pt surface. Herein, these issues are well addressed by design and synthesis of effective catalyst binders based on polymers of intrinsic microporosity (PIMs) with strong hydrogen-bonding functionalities which improve phosphoric acid binding energy, and thus preferably uphold phosphoric acid in the vicinity of Pt catalyst particles to mitigate the adsorption of phosphoric acid on the Pt surface. With combination of the highly mass transport microporosity, strong hydrogen-bonds and high phosphoric acid binding energy, the tetrazole functionalized PIM binder enables an H2-O2 cell to reach a high Pt-mass specific peak power density of 3.8 W mgPt−1 at 160 °C with a low Pt loading of only 0.15 mgPt cm−2.

This communication represents an interesting addition to HT-PEM FC research field. The topic is novel and relevant. In particular, the optimisation of binders for this FC type is often omitted for the favour of membranes. Study covers preparation of binders, their implementation into electrodes followed by detailed characterisation and single cell testing. Authors demonstrated promising results and supported them with sufficient data. However, there are certain issues to be addressed before publication.
General remarks: There is no heading indicating the start of "Results and Discussion" section.
There is a high number of typing errors in manuscript.
Readability of text is not ideal, especially due to vast number of experiments with various parameters. It is not entirely clear what were conditions and materials used for each experiment shown in figures. Description of figures is often misleading. Authors may consider adding a table, specifying performed single cell experiments. In general, structure of experimental part can be significantly improved and separated into parts corresponding to the ones in results and discussion section.
Authors often discuss in text Pt utilisation, which they determine based on size of Pt nanoparticles in catalyst. However, there are certain problems connected to their experimental procedure.
Voltammograms measured with anode as hydrogen pseudo-reference electrode will never provide exact information about electrochemically active surface area on the cathode and absolute values are almost never correct, especially in system with strongly adsorbing electrolyte. Relative comparison is correct, but then the exact info about the catalyst is needed, which is missing in study.
Electrochemical stability of PBI binder is often an issue (10.1149/2.0741506jes). Did authors consider performing electrochemical experiments to verify the stability of PIM-based binders?
Specific comments: Line 32 -What authors mean by strong adsorption of phosphoric acid molecules at low potentials and acid anions at high potentials? This is essentially wrong, because at low potentials corresponding to HUPD, phosphate anions are mostly removed/reduced to phosphorous acid and at high potentials surface is mostly covered by adsorbed O. This part of text needs clarification according to potential ranges. Authors may consider adding references as for example 10.1021/jp311924q, 10.1016/j.electacta.2015.01.097, 10.1002/celc.201300134.
Line 64 -Have authors considered also binders based on ion-pairs?
Line 227 -Is hydrogen desorption region really well suited for determination of electrochemically active surface area? What about adsorption region? Line 280, Figure 4 -Results in this figure does not seemingly match the impedance spectra provided in supplementary materials, Figure 6, although it seems that conditions are the same. In Figure 4, fuel cell using PTFE and PBI binders exhibit the lowest HFR, but that is not the case in Supplementary Figure 6, where PTFE has approximately the same resistance as PIM-1 binder. In addition, impedance spectra for PBI and best binder PIM-Tz are almost identical. Ascribing polarisation resistance to mass-transfer related phenomena is essentially incorrect and spectra are not fitted. This presents a major problem, as in present form, these data does not support conclusions. Authors may consider checking their data and fit the spectra. That way, the impact of binder on anodic and cathodic reaction can be evaluated as well.
Line 365 -Was commercial GDE used in work?
Line 413 -it is not clear for which purpose these membranes were prepared. Was it for the determination of permeability or for MEA preparation?
Line 517 -Is there a reason why two different potentiostats were used in study?

Response to Reviewers' comments:
Reviewer #1 (R1): The manuscript "High Pt Utilization Enabled by Hydrogen-bonding Microporous Polymer Binders for High-

How about the cell performance if the catalyst loading is further reduced?
A1-1 Response: Further reduction of the Pt loading is also studied, which gives much lower cell performance. Enhancement of the FC performance is possible but needs further optimization of the catalyst layer, which will be the future work. of 30 m 2 /g. This material is very brittle due to the abundant intramolecular hydrogen bonds. We have tried to use it as the binder in the catalyst layer for HT-PEMFC, however, its low porosity and hence low gas permeability lead to a poor cell performance. It is therefore that a relatively low (75%) functionalization degree is used in the present work. This comments is addressed by adding te following lines in the revised mansucript: Line 126-129: "PIM-Tz with DF100% with a BET area of 30 m 2 /g was initially tested which showed poor performance due to the strong hydrogen bonding and the resultant low porosity or gas permeability. In the following work a relatively low (75%) DF is used for PIM-Tz." A1-3 Response to Minor points: The manuscript has been polished and small changes throughout the manuscript are made including editorial format and linguistic corrections.

Reviewer #2 (R2):
The current manuscript is an interesting one and deals with one of the critical problem for any PEM fuel cell, particularly high-temp PEM based FC. Author has demonstrated the work very well. I recommend publication with the following corrections, which may be included: A2-0 Response: We are very grateful to the reviewer for the positive comments.
1. The author has described Pt loading in some cases as "mgPt cm-2" and in some other cases as "mg cm-2". These different representations of Pt loading should be avoided.

A2-1 Response:
The Pt loading has been uniformaly expressed as "mgPt cm -2 ". This information is added to the revised manuscript as follow: Line 45-46, "The Pt loading is normally in a high metal 40-60 wt%Pt range in order to reduce the over catalyst layer thickness and hence the mass transportation limitation."

In line 75, the author has mentioned 'The best binder' which may confuse the reader. Please mention it
correctly by reconstructing the sentence.

A2-3 Response:
The wording "the best binder" has been changed to "the candidate binder" in the revised manuscript.

In line 88, the author has mentioned 'ENREF 24' in the sentence which is not clear. Please recheck it.
A2-4 Response: This is an error and " ENREF 24" has been deleted.

5.
In line 115, degree of functionalization was mentioned as 100% but no supporting data or the calculation procedure is available. Please mention the instrumentation technique used to determine such factor. Does it represent the degradation of the PIM-1? A2-6 Response: During the modification process, PIM-1 degrades due to the severe reaction conditions e.g. strong acidity or high temperature as shown in the experimental part in the manuscript, thus leading to a drop of the molecular weight of the functional PIMs. This information is added to the revised manuscript: Line 129-131: "During the modification process, PIM-1 degrades due to the severe reaction conditions e.g. strong acidity or high temperature as indicated by the slightly decreased molecular weight ranging from 40 to 53 kDa (Table 1) General remarks: 1. There is no heading indicating the start of "Results and Discussion" section.
A3-1 Response: The head of "Results and Discussion" has been added in the second part in the manuscript.

There is a high number of typing errors in manuscript.
A3-2 Response: See Response to R1-3 above (The manuscript has been polished and small changes throughout the manuscript are made including editorial format and linguistic corrections.) The cell test conditions

Authors often discuss in text Pt utilization, which they determine based on size of Pt nanoparticles in catalyst.
However, there are certain problems connected to their experimental procedure. Voltammograms measured with anode as hydrogen pseudo-reference electrode will never provide exact information about electrochemically active surface area on the cathode and absolute values are almost never correct, especially in system with strongly adsorbing electrolyte. Relative comparison is correct, but then the exact info about the catalyst is needed, which is missing in study.

A3-5 Response:
We do agree with the view that the cyclic voltammograms provide a rough estimation of the electrochemically active surface area (ECSA) of the cathode catalysts. The "Pt utilization" based on the ECSA is therefore a poorly defined term. We have therefore revised the manuscript by using "the catalyst performance" instead of "the Pt utilization" in a general sense. In specific cases the term "the Pt-mass specific peak power density, which is directly obtainable from the cell performance and electrode Pt loading, is used instead of "the Pt utilization". However, the content of the electrochemically active surface area (ECSA) is kept for relative/qualitative comparison of electrodes with different binder materials.
Accordingly, the title of the manuscript has been changed as "Low Pt Loading for High-performance Fuel Cell Electrodes Enabled by Hydrogen-bonding Microporous Polymer Binders".
In addition, more specific information on the catalyst is given in the Experimental Materials part, Line 384: The reversal potential is a critical issue for the hydrogen adsorption region as the hydrogen evolution reaction may be involved at potentials near the reversible hydrogen reduction potential (0.0 V). In case of phosphoric acid electrolyte or Pt alloy catalysts, the CO stripping method is more appropriate, which is however not available for the present study. No action is taken in connection to this comment.
9. Line 280, Figure 4 -Results in this figure does not seemingly match the impedance spectra provided in supplementary materials, Figure 6, although it seems that conditions are the same. In Figure 4, fuel cell using PTFE and PBI binders exhibit the lowest HFR, but that is not the case in Supplementary Figure 6 That way, the impact of binder on anodic and cathodic reaction can be evaluated as well.
A3-9 Response: In a qualitative manner we understood the EIS in the following ways: 1) The HFR originates from the ohmic resistance of the membrane, which is determined by the amount of the doping acid.
When a fuel cell is assembled and activated (during the breakin period) the doping acid transfers from the membrane to the catalyst layer. For hydrophobic (with weaker acid affinity) binders (PTFE and PIM-1) less acid is absorbed by the catalyst layer i.e. more acid remains in the membrane. As a result, the HFR due to the membrane resistance will be smaller while the kinetic resistance will be larger. The same arguments apply to the hydrophilic binders (PIM-Tz, mPBI, etc) with opposite behaviors. This is consistent with both the I-V were used in this work, and the H2-air cell performance was listed in the Supplementary Table 3 in the SI. 11. Line 413 -it is not clear for which purpose these membranes were prepared. Was it for the determination of permeability or for MEA preparation?
A3-11 Response: Yes, the purpose to prepare PIMs membranes is for determination of the gas permeability of the bulk polymer materials themselves, which is indicative to the gas permeability of the corresponding GDEs.

Line 517 -Is there a reason why two different potentiostats were used in study?
A3-12 Response: All the electrochemical performances were measured by a Bio-Logic VSP-300 potentiostat. The fuel cell workstation (Smart 2-WonATech Inc., Korea) was employed to check the repeatability of the cell performance. The corresponding description has been changed in the revised manuscript as follows: Line 597: "……A fuel cell workstation (Smart 2-WonATech Inc., Korea) was employed to check the repeatability of the cell performance……"

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): The revision article has been revised in detail in accordance with the revision opinions, which can be accepted and published at present.
Reviewer #2 (Remarks to the Author): Author has revised the manuscript quite well by responding to all the questions raised by the reviewers Reviewer #3 (Remarks to the Author): I would like to thank authors for addressing all comments, their effort is well-appreciated. In my opinion, reviewed version of manuscript meets high standards of journal and should be published.